# Patent

## Title
Electric power generation method using thermoelectric power generation element, thermoelectric power generation element and method of producing the same, and thermoelectric power generation device

## Abstract

The present invention provides an electric power generation method using a thermoelectric power generation element, a thermoelectric power generation element, and a thermoelectric power generation device, each of which has higher thermoelectric power generation performance than conventional ones and can be used for more applications. The thermoelectric power generation element includes a first electrode and a second electrode that are disposed to oppose each other, and a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes, where the laminate has a structure in which Bi2Te3 layers and metal layers containing Al, Cu, Ag, or Au are laminated alternately, a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1, lamination surfaces of the Bi2Te3 layers and the metal layers are inclined at an inclination angle θ of 15° to 60° with respect to a direction in which the first electrode and the second electrode oppose each other, and a temperature difference applied in a direction perpendicular to the direction in the element generates a potential difference between the first and second electrodes. The electric power generation method and thermoelectric power generation device each use the element.

## Background
RELATED APPLICATIONS
This application is a continuation of U.S. National Phase under 35 U.S.C. 371 of International Application No. PCT/JP2007/068203, filed on Sep. 19, 2007, which in turn claims the benefit of Japanese Application No. 2006-323381, filed on Nov. 30, 2006, the disclosures of which Applications are incorporated by reference herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric power generation method using a thermoelectric power generation element, which is a method of obtaining electrical energy directly from thermal energy. Furthermore, the present invention also relates to a thermoelectric power generation element that converts thermal energy directly into electrical energy and the method of producing the same, as well as a thermoelectric power generation device.
2. Description of the Related Art
Thermoelectric power generation is a technique for converting thermal energy directly into electrical energy by utilizing the Seebeck effect whereby an electromotive force is generated in proportion to the temperature difference applied to both ends of a material. This technique is used practically in, for example, remote area power supply, space power supply, and military power supply.
A conventional thermoelectric power generation element generally has a structure that is referred to as a so-called “π-type structure”, in which a “p-type semiconductor” and an “n-type semiconductor” that are different in carrier sign from each other are combined together thermally in parallel and electrically in series.
Generally, the performance of a thermoelectric material that is used for a thermoelectric power generation element is evaluated by a figure of merit Z or a figure of merit ZT nondimensionalized by multiplying Z by absolute temperature. ZT can be expressed by a formula of ZT=S2/ρκ, where S denotes the Seebeck coefficient of a thermoelectric material, ρ indicates electrical resistivity, and κ is thermal conductivity. Furthermore, S2/ρ, which is an index expressed with consideration being given to only the Seebeck coefficient S and electrical resistivity ρ, also is referred to as a power factor (output factor) and serves as a criterion for evaluating the power generation performance of a thermoelectric material obtained when the temperature difference is constant.
Bi2Te3 that currently is used practically as a thermoelectric material has a relatively high thermoelectric power generation performance, specifically, a ZT of approximately 1 and a power factor of about 40 to 50 μW/(cm·K2). However, in the case of an element having the aforementioned π-type structure, it is difficult to obtain a high thermoelectric power generation performance. Thus, it has not reached a level that is sufficiently high enough to allow it to be used practically for more various applications.
On the other hand, there has been a proposal for an element utilizing anisotropy of the thermoelectric properties in a laminated structure that is naturally-occurring or is produced artificially as an element having a different structure from the π-type structure (Thermoelectrics Handbook, Chapter 45 “Anisotropic Thermoelements”, CRC Press (2006): Reference 1). However, according to Reference 1, it is difficult to improve ZT of such an element. Therefore, it is developed not for the use for thermoelectric power generation but for assumed uses mainly in the field of measurement such as an infrared sensor.
Furthermore, JP 6(1994)-310766 A (Reference 2) discloses, as a thermoelectric material having a similar structure thereto, a material in which a material having thermoelectric properties, which is typified by Fe—Si materials, and an insulating material with a thickness of 100 nm or less, which is typified by SiO2, are arranged alternately in the form of stripes on a substrate. According to Reference 2, in a material having such a microstructure, as compared to the case where a Fe—Si material having thermoelectric properties is used independently, the Seebeck coefficient S can be improved but on the other hand, the electrical resistivity ρ increases due to the insulating material contained therein. Accordingly, the element made thereof has increased internal resistance, and the electric power obtained therewith is reduced conversely.
Examples of other thermoelectric materials having laminated structures include a material having a layered body formed of semimetal, metal, or synthetic resin, which is disclosed in WO 00/076006 (Reference 3). This material is based on the configuration in which, as in the case of the conventional π-type structure, a temperature difference is applied to the direction in which the respective layers of the layered body are laminated, and thereby electric power is extracted through a pair of electrodes that are disposed so as to oppose in the same direction as that described above. Therefore the element disclosed in Reference 3 is substantially different from that disclosed in Reference 1.
SUMMARY OF THE INVENTION
As described above, in conventional thermoelectric materials, it is not possible to obtain thermoelectric power generation performance that is sufficiently high enough to allow them to be used practically for more various applications. The present inventors made studies assiduously with respect to the thermoelectric power generation element formed using a laminate. As a result, they obtained the following unexpected findings to reach the present invention based thereon. That is, a laminate formed of a Bi2Te3 (bismuth telluride) layer and a metal layer containing a specific metal was used, with the thickness ratio between the Bi2Te3 layer and the metal layer being in a specific range, the lamination surfaces of the laminate were inclined at a predetermined inclination angle θ with respect to the direction in which electrodes, between which the laminate was interposed, oppose each other, and thereby, as compared to the case where Bi2Te3 was used independently as a thermoelectric material, the power factor of the element was increased and the thermoelectric power generation characteristics were improved considerably.
That is, an electric power generation method using a thermoelectric power generation element of the present invention is a method for obtaining electric power from the element by applying a temperature difference in the thermoelectric power generation element. In this method, the element includes a first electrode and a second electrode that are disposed to oppose each other, and a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes, the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately, a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1, and lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to the direction in which the first electrode and the second electrode oppose each other. The method includes a step of applying a temperature difference in the direction perpendicular to the direction in which the first electrode and the second electrode oppose each other in the element, so that electric power is obtained through the first and second electrodes.
The thermoelectric power generation element of the present invention includes a first electrode and a second electrode that are disposed to oppose each other and a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes, where the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately, a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1, lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to the direction in which the first electrode and the second electrode oppose each other, and a temperature difference applied in the direction perpendicular to the direction in which the first electrode and the second electrode oppose each other in the element generates a potential difference between the first and second electrodes.
A method of producing a thermoelectric power generation element of the present invention is a method of producing a thermoelectric power generation element that includes a first electrode and a second electrode that are disposed to oppose each other and a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes, where the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately, a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1, lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to the direction in which the first electrode and the second electrode oppose each other, and a temperature difference applied in the direction perpendicular to the direction in which the first electrode and the second electrode oppose each other in the element generates a potential difference between the first and second electrodes, wherein the method includes cutting out an original plate, in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately and a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1, in a direction that obliquely traverses lamination surfaces of the Bi2Te3 layer and the metal layer, and disposing the first and second electrodes on the laminate thus obtained so that the first and second electrodes oppose each other and the direction in which they oppose each other traverses the lamination surfaces at an inclination angle θ of 15° to 60°.
A thermoelectric power generation device of the present invention includes a support plate and a thermoelectric power generation element disposed on the support plate, where the element includes first and second electrodes that are disposed to oppose each other, and a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes, the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately, a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1, lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to the direction in which the electrodes of a pair oppose each other, the element is disposed on the support plate in such a manner that the direction perpendicular to the direction in which the electrodes of a pair oppose each other agrees with the direction perpendicular to the surface of the support plate on which the element is disposed, and a temperature difference is applied in the direction perpendicular to the surface of the support plate, so that electric power is obtained through the electrodes of a pair.
According to the electric power generation method, thermoelectric power generation element, and thermoelectric power generation device of the present invention, higher thermoelectric power generation characteristics can be obtained as compared to conventional thermoelectric power generation methods, thermoelectric power generation elements, and thermoelectric power generation devices (typically, thermoelectric power generation methods, thermoelectric power generation elements, and thermoelectric power generation devices in which Bi2Te3 is used independently as a thermoelectric material). The present invention improves the efficiency of energy conversion between thermal energy and electrical energy and has an effect of facilitating application of thermoelectric power generation to various fields and thus has an industrially high value.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing an example of a thermoelectric power generation element according to the present invention as well as the direction in which first and second electrodes oppose each other, the direction in which a temperature difference is to be applied, and an inclination angle θ.
FIG. 2 is a schematic view showing an example of the configuration for driving the thermoelectric power generation element of the present invention.
FIG. 3 is a schematic view showing an example of the method of cutting out a laminate from an original plate in the method of producing a thermoelectric power generation element of the present invention.
FIG. 4 is a perspective view that schematically shows an example of the thermoelectric power generation device of the present invention.
FIG. 5 is a perspective view that schematically shows another example of the thermoelectric power generation device of the present invention.



BEST MODE FOR CARRYING OUT THE INVENTION
In the electric power generation method of the present invention, the lamination surfaces of the Bi2Te3 layer and the metal layer in the laminate may have an angle (inclination angle) θ of 25° to 40°, at which they are inclined with respect to the direction in which the first electrode and the second electrode oppose each other.
In the electric power generation method of the present invention, the meta layer contains preferably Cu, Ag, or Au and more preferably Cu or Ag.
In the electric power generation method of the present invention, it is preferable that the thickness ratio between the metal layer and the Bi2Te3 layer be in the range of metal layer:Bi2Te3 layer=100:1 to 80:1.
In the electric power generation method of the present invention, the power factor of the element may be at least 200 (μW/(cm·K2)).
In the electric power generation method of the present invention, the lamination surfaces of the Bi2Te3 layer and the metal layer in the laminate may have an angle (inclination angle) θ of 25° to 40°, at which they are inclined with respect to the direction in which the first electrode and the second electrode oppose each other. The metal layer may contain Cu or Ag, the thickness ratio between the metal layer and the Bi2Te3 layer may be in the range of metal layer:Bi2Te3 layer=100:1 to 80:1, and in this case, the element may have a power factor of at least 800 (μW/(cm·K2)).
In the thermoelectric power generation element of the present invention, the lamination surfaces of the Bi2Te3 layer and the metal layer in the laminate may have an angle (inclination angle) θ of 25° to 40°, at which they are inclined with respect to the direction in which the first electrode and the second electrode oppose each other.
In the thermoelectric power generation element of the present invention, the metal layer contains preferably Cu, Ag, or Au and more preferably Cu or Ag.
In the thermoelectric power generation element of the present invention, the thickness ratio between the metal layer and the Bi2Te3 layer is preferably in the range of metal layer:Bi2Te3 layer=100:1 to 80:1.
In the thermoelectric power generation element of the present invention, the element may have a power factor of at least 200 (μW/(cm·K2)).
In the thermoelectric power generation element of the present invention, the lamination surfaces of the Bi2Te3 layer and the metal layer in the laminate may have an angle (inclination angle) θ of 25° to 40°, at which they are inclined with respect to the direction in which the first electrode and the second electrode oppose each other, the metal layer may contain Cu or Ag, the thickness ratio between the metal layer and the Bi2Te3 layer may be in the range of metal layer: Bi2Te3 layer=100:1 to 80:1, and in this case, the element may have a power factor of at least 800 (μW/(cm·K2)).
The thermoelectric power generation device of the present invention may include at least two of the aforementioned thermoelectric power generation elements, and in this case, the elements may be connected electrically in series with each other through the electrodes or may be connected electrically in parallel with each other through the electrodes.
<Thermoelectric Power Generation Element>
FIG. 1 shows an example of the thermoelectric power generation element of the present invention. The thermoelectric power generation element 1 shown in FIG. 1 includes a first electrode 11 and a second electrode 12 that are disposed to oppose each other and a laminate 13 that is interposed between the first electrode 11 and the second electrode 12 and is electrically connected to both the electrodes. The laminate 13 is connected to principal surfaces of the first electrode 11 and the second electrode 12, and the principal surfaces of both the electrodes are in parallel with each other. The shape of the laminate 13 shown in FIG. 1 is rectangular parallelepiped, and the first electrode 11 and the second electrode 12 are disposed on a pair of opposing surfaces thereof. The surfaces of the first and second electrodes are orthogonal to the direction (opposing direction 17) in which the first and second electrodes oppose each other.
The laminate 13 has a structure in which Bi2Te3 layers 14 and metal layers 15 containing Al, Cu, Ag, or Au are laminated alternately. The lamination surfaces of the respective layers (the direction 16 that is in parallel with the principal surface of each layer) are inclined at an inclination angle θ of 15° to 60° with respect to the opposing direction 17. The thickness ratio between a metal layer 15 and a Bi2Te3 layer 14 in the laminate 13 is in the range of metal layer:Bi2Te3 layer=400:1 to 20:1.
In the element 1, the temperature difference applied in the direction 18 perpendicular to the opposing direction 17 generates a potential difference between the first electrode 11 and the second electrode 12. In other words, a temperature difference is applied in the direction 18 perpendicular to the opposing direction 17 in the element 1, so that electric power can be extracted through the first electrode 11 and the second electrode 12.
Specifically, for example, as shown in FIG. 2, a temperature difference is applied to the direction 18 perpendicular to the opposing direction 17 in which the electrodes 11 and 12 oppose each other, with a hot section 22 being attached closely to one surface of the laminate 13 of the element 1 where the electrodes 11 and 12 are not disposed and a cold section 23 being attached closely to the other surface, thereby a potential difference is generated between the electrodes 11 and 12, and thus electric power can be extracted through both the electrodes. On the other hand, in a conventional thermoelectric power generation element having a π-type structure, electromotive force is generated only in the direction parallel to the direction in which the temperature difference is applied and is not generated in the direction perpendicular thereto. Accordingly, in the conventional thermoelectric power generation element, it is necessary to apply a temperature difference between the pair of electrodes, through which electric power is extracted. In the element 1, both the opposing direction 17 in which the first electrode 11 and the second electrode 12 oppose each other and the direction 18 in which the temperature difference is applied traverse the lamination surfaces of the respective layers of the element 13. Furthermore, the direction 18 in which the temperature difference is applied is not limited as long as it is substantially perpendicular to the opposing direction 17 in which the electrodes 11 and 12 oppose each other (similarly in this specification, the term “perpendicular” embraces “substantially perpendicular”).
Conventionally, as disclosed in Reference 2, it has been difficult to improve both the Seebeck coefficient S and the electrical resistivity ρ of the thermoelectric material and to increase the power factor of the element. However, in the element 1, as compared to the case where Bi2Te3 is used independently as the thermoelectric material, the power factor of the element can be increased and high thermoelectric power generation characteristics can be obtained.
The composition of bismuth telluride composing the Bi2Te3 layer 14 may deviate from the composition expressed by a formula of Bi2Te3 depending on the production conditions. The composition of bismuth telluride composing the Bi2Te3 layer 14 is not limited as long as it satisfies a range of 2<X<4 in terms of Bi2TeX.
The metal layer 15 contains Al, Cu, Ag, or Au. The metal layer 15 contains preferably Cu, Ag, or Au and particularly preferably Cu or Ag. In this case, higher thermoelectric power generation characteristics can be obtained. The metal layer 15 may contain such metal independently or as an alloy. When the metal layer 15 contains such metal independently, the metal layer 15 is composed of Al, Cu, Ag, or Au, preferably Cu, Ag, or Au, and particularly preferably Cu or Ag.
Preferably, a material with excellent electroconductivity is used for the first electrode 11 and the second electrode 12. For example, it also may be metal such as Cu, Ag, Mo, W, Al, Ti, Cr, Au, Pt, or In, or a nitride or oxide such as TiN, indium tin oxide (ITO), or SnO2. Furthermore, for example, a solder, a silver solder, or an electroconductive paste also can be used for the electrodes.
Although the detail is described in the section of Example, the present inventors studied various conditions and found out that the power factor of the element 1 further was improved and higher thermoelectric power generation characteristics were obtained by controlling the inclination angle θ formed between the lamination surfaces of the respective layers of the laminate 13 and the opposing direction 17 in which the electrodes 11 and 12 oppose each other, and the thickness ratio between the Bi2Te3 layer 14 and the metal layer 15.
The inclination angle θ is 15° to 60°, preferably 25° to 40°.
The thickness ratio between the metal layer 15 and the Bi2Te3 layer 14 is in the range of metal layer:Bi2Te3 layer=400:1 to 20:1, preferably in the range of metal layer:Bi2Te3 layer=100:1 to 80:1.
From the viewpoint of the combination of the inclination angle θ, the type of the metal layer 15, and the thickness ratio, it is more preferable that the inclination angle θ be 25° to 40°, the metal layer 15 contain Cu or Ag, and the thickness ratio between the metal layer 15 and the Bi2Te3 layer 14 be in the range of metal layer:Bi2Te3 layer=100:1 to 80:1.
Depending on these conditions, the power factor (output factor) of the element 1 can be at least 200 (μW/(cm·K2)), further can be at least 400 (μW/(cm·K2)), at least 500 (μW/(cm·K2)), at least 600 (μW/(cm·K2)), at least 700 (μW/(cm·K2)), or at least 800 (μW/(cm·K2)).
<Method of Producing Thermoelectric Power Generation Element>
The thermoelectric power generation element 1 can be formed as follows. That is, for example, as shown in FIG. 3, an original plate (laminated original plate) 34, in which Bi2Te3 films 31 and metal films 32 containing Al, Cu, Ag, or Au are laminated alternately and the thickness ratio between a metal film 32 and a Bi2Te3 film 31 is in the range of metal film:Bi2Te3 film=400:1 to 20:1, is cut out in a direction that obliquely traverses the lamination surfaces 35 of the Bi2Te3 films 31 and the metal films 32 (for example, is cut out in such a manner that the angle formed between the cut out face and the lamination surfaces 35 is 15° to 60°), and the first and second electrodes are disposed on the resultant laminate (13a, 13b, 13c, or 13d) so as to oppose each other and so that the direction in which they oppose each other traverses the lamination surfaces 35 at an inclination angle θ of 15° to 60°. Numeral 33 denotes a laminate 33 that was obtained by cutting out the original plate 34 so as to traverse the lamination surfaces 35 perpendicularly thereto. The thermoelectric power generation element of the present invention cannot be formed from such a laminate. The phrase “the first and second electrodes are disposed so that the direction in which they oppose each other traverses the lamination surfaces 35” denotes that, for example, with respect to the laminate 13d shown in FIG. 3, the electrodes are disposed on the side faces A and A′ or the side faces B and B′.
The metal film 32 may be formed of metal that is identical to that composing the metal layer 15.
The original plate 34 can be formed by superimposing a plurality of metal films 32 (typically, metal foils), each of which has a Bi2Te3 film formed on the surface thereof and bonding them together under pressure. At the time of bonding under pressure, heat may be applied in addition to pressure. The Bi2Te3 film may be formed on one surface of the metal film 32 or may be formed on each surface thereof. However, when using the metal film 32 with the Bi2Te3 film formed on each surface thereof, the degree of adhesion between the respective layers that compose the original plate 34 can be improved.
Furthermore, for example, the original plate 34 also can be formed by depositing the Bi2Te3 films 31 and the metal films 32 alternately.
Formation of the Bi2Te3 film on the surface of the metal film 32 and deposition of the Bi2Te3 film 31 and the metal film 32 can be carried out by various thin film forming methods, for example, a sputtering method, an evaporation method, a laser ablation method, a vapor deposition methods including a chemical vapor deposition method, a liquid phase growth method, or a plating method. The thickness ratio between the Bi2Te3 film 31 and the metal film 32 that are formed by any one of the above-mentioned thin film formation techniques may be adjusted by a general method.
A known method such as a cutting process may be used for cutting out the original plate 34. The surfaces of the laminate 13 obtained by cutting out may be polished if necessary.
When the first and second electrodes are to be disposed, it is not always necessary to dispose the electrodes on the whole surfaces of the laminate 13 on which the electrodes are to be disposed. The electrodes may be disposed on parts of the surfaces of the laminate 13 on which the electrodes are to be disposed.
The method of disposing the first and second electrodes is not particularly limited and various thin film formation techniques such as a sputtering method, an evaporation method, and a vapor growth method, or techniques of applying an electroconductive paste, plating, or spraying can be used. For example, electrodes formed separately may be joined to the laminate 13 with, for example, a solder or a silver solder.
The thermoelectric power generation element 1 also can be produced by methods other than those described above. For instance, an etching mask having openings periodically is placed on the surface of a metal plate containing Al, Cu, Ag, or Au, and the surface of the metal plate is irradiated with etching particles that have high linearly advancing properties from the direction oblique thereto and thereby the metal plate is formed, with slits inclined with respect to the surface thereof being arranged at equal intervals in the cross section thereof. Subsequently, Bi2Te3 is deposited inside the slits (for example, the insides of the slits are subjected to vapor deposition or plating with Bi2Te3), so that laminates 13 may be formed. First and second electrodes are disposed with respect to each laminate 13 in the same manner as described above, and thus the thermoelectric power generation element 1 can be formed.
<Thermoelectric Power Generation Device>
FIG. 4 shows an example of the thermoelectric power generation device of the present invention. The device 41 shown in FIG. 4 includes a support plate 45 and six thermoelectric power generation elements 1 of the present invention disposed on the support plate 45. Each element 1 is disposed on the support plate 45 in such a manner that the direction perpendicular to the direction 17 in which the first and second electrodes oppose each other in each element agrees with the direction perpendicular to the surface 46 of the support plate 45 on which the elements 1 are disposed. Furthermore, adjacent elements 1 are connected electrically in series with each other through a connecting electrode 43 that also serves as the first or second electrode of each element 1. Extraction electrodes 44, each of which also serves as the first or second electrode, are disposed in elements 1a and 1b located on the ends of the sequence of the six elements 1.
In the device 41, a temperature difference is allowed to be applied in the direction perpendicular to the surface 46 of the support plate 45. For example, a cold section is brought into contact with the surface of the support plate 45 on which the elements 1 are not disposed, a hot section is brought into contact with the opposite surface to the surface of the element 1 that is in contact with the support plate 45, and thereby electric power can be obtained through the extraction electrodes 44. In the example shown in FIG. 4, in the adjacent elements 1, the directions in which the lamination surfaces of the Bi2Te3 layers and the metal layers are inclined are opposite to each other. This is intended to prevent the electromotive force generated in the elements 1 due to the application of the temperature difference from being cancelled between the adjacent elements 1.
FIG. 5 shows another example of the thermoelectric power generation device of the present invention. The device 42 shown in FIG. 5 includes a support plate 45 and eight thermoelectric power generation elements 1 of the present invention disposed on the support plate 45. Each element 1 is disposed on the support plate 45 in such a manner that the direction perpendicular to the direction 17 in which the first and second electrodes oppose each other in each element agrees with the direction perpendicular to the surface 46 of the support plate 45 on which the elements 1 are disposed. The eight elements 1 are divided into four blocks that are disposed on the support plate 45, with one block including two elements 1. Elements of one block (for example, element 1a and 1b) are connected electrically in parallel with each other through a connecting electrode 43 that also serves as the first or second electrode of each element. The blocks adjacent to each other are connected electrically in series through the connecting electrodes 43.
In the device 42, a temperature difference is allowed to be applied in the direction perpendicular to the surface 46 of the support plate 45. For example, a cold section is brought into contact with the surface of the support plate 45 on which the elements 1 are not disposed, a hot section is brought into contact with the opposite surface to the surface of the element 1 that is in contact with the support plate 45, and thereby electric power can be obtained through the extraction electrodes 44. In the example shown in FIG. 5, the directions in which the Bi2Te3 layers and the metal layers are inclined are identical to each other in the elements 1 included in one block, and they are opposite to each other in the adjacent blocks. This is intended to prevent the electromotive force generated in the elements 1 due to the application of the temperature difference (generated in the blocks due to the application of the temperature difference) from being cancelled between the adjacent elements 1 and between the adjacent blocks.
The configuration of the thermoelectric power generation device of the present invention is not limited to the examples shown in FIGS. 4 and 5. For example, one thermoelectric power generation element may be disposed on the support plate. However, when the thermoelectric power generation device is formed with at least two thermoelectric power generation elements being disposed as in the examples shown in FIGS. 4 and 5, more electrical energy can be obtained. Furthermore, as in the example shown in FIG. 4, when the elements are connected electrically in series with each other, the voltage obtained is increased. As in the example shown in FIG. 5, when the elements are connected electrically in parallel with each other, the possibility that the function of the thermoelectric power generation device as a whole can be maintained even in the case where the electrical connection of the elements 1 is lost partially can be increased and thus the reliability of the thermoelectric power generation device can be improved. That is, a suitable combination of the series and parallel connections of the elements makes it possible to configure a thermoelectric power generation device with high thermoelectric power generation characteristics.
The structures of the connecting electrodes 43 and the extraction electrodes 44 are not particularly limited as long as they are excellent in electroconductivity. For example, the connecting electrodes 43 and the extraction electrodes 44 may be formed of metal such as Cu, Ag, Mo, W, Al, Ti, Cr, Au, Pt, or In, or nitride or oxide such as TiN, indium tin oxide (ITO), or SnO2. Furthermore, a solder, a silver solder, or an electroconductive paste also can be used for the electrodes.
<Electric Power Generation Method Using Thermoelectric Power Generation Element>
The electric power generation method of the present invention is a method of obtaining electric power through a first electrode 11 and a second electrode 12 (or connecting electrodes 43 or extraction electrodes 44), by applying a temperature difference in the direction perpendicular to the opposing direction 17 in which the electrodes oppose each other in a thermoelectric power generation element 1 of the present invention described above.
EXAMPLE
Hereinafter, the present invention is described in further detail. The present invention is not limited to the following examples.
Example 1
In Example 1, thermoelectric power generation elements 1 as shown in FIG. 1 were produced using Bi2Te3 and several types of metals (Au, Ag, Cu and Al), and then the thermoelectric power generation characteristics thereof were evaluated.
First, a metal foil (Au foil, Ag foil, Cu foil, or Al foil) with a size of 100 mm×100 mm and a thickness of 99 μm was prepared, and a 0.5-μm thick Bi2Te3 film was formed on each surface of the metal foil by an electrolytic plating method.
Formation of each Bi2Te3 film by the electrolytic plating method was carried out as follows. First, Bi2O3 and TeO2, raw materials for Bi2Te3, were dissolved in a nitric acid aqueous solution so that a ratio of Bi:Te (mole ratio)=1:1 was obtained and thus an electrolyte was prepared. Subsequently, in the electrolyte thus prepared, the metal foil and a counter electrode formed of platinum were disposed. Next, with Ag/AgCl disposed in a KCl saturated solution being used as a reference electrode, a potential difference of 50 mV was applied between the metal foil and the counter electrode with a potentiostat while being monitored. Thus the electrolytic plating as described above was carried out. In this case, the metal foil serves as a working electrode. The electrolyte was stirred and was subjected to bubbling of nitrogen gas during the formation of the Bi2Te3 film by electrolytic plating.
The composition of the Bi2Te3 film formed as described above was evaluated by an energy dispersive X-ray analysis method (EDX). As a result, it was confirmed that the composition of the film was Bi2Te2.5 and the film contained Bi in excess as compared to Bi2Te3. Furthermore, the crystal structure of the film was evaluated by an X-ray diffraction method (XRD). As a result, the crystal structure of the film was identical to that of Bi2Te3.
Next, a sheet of a Bi2Te3 film/a metal foil/a Bi2Te3 film formed as described above was cut into a size of 5 mm×50 mm to form small strip pieces. Two hundred small pieces thus formed were superimposed together and were subjected to heat pressure bonding at 250° C. for one hour under a reduced pressure of 10−4 Pa while a load of 100 kg/cm2 was applied in the direction in which they were laminated. Thereafter, it was subjected to cutting and polishing and thus a laminated original plate with a size of 3 mm×48 mm and a thickness of 20 mm was obtained. The section of the original palate thus obtained was observed with a scanning electron microscope (SEM). As a result, it was observed that metal layers (derived from the metal foils), each of which had a thickness of about 99 μm, and Bi2Te3 layers (derived from Bi2Te3 films), each of which had a thickness of about 1 μm, were laminated alternately.
The laminate 13 with a thickness of 1 mm, a width of 3 mm, and a length of 20 mm was cut out from the original plate obtained as described above by cutting with a diamond cutter as shown in FIG. 3, with the inclination angle θ being changed at 15° intervals from 0° to 90°. Thereafter, a first electrode 11 and a second electrode 12 made of Au were formed, by the sputtering method, on the end faces (corresponding to the side faces B and B′ shown in FIG. 3) located in the direction of the long side of each laminate 13 cut out as described above. Thus each thermoelectric power generation element 1 as shown in FIG. 1 was obtained.
Next, as shown in FIG. 2, one surface of the element 1 on which the electrodes were not disposed was heated to 150° C. with a heater and the surface opposing thereto was maintained at 30° C. by water-cooling. Thus, a temperature gradient was applied in the direction perpendicular to the opposing direction 17, and the voltage (electromotive voltage) generated between the electrodes thereby and the electrical resistance value obtained between the electrodes were measured. Thus, the power factor of the element 1 was determined. The direction in which the temperature gradient was applied was the direction that traversed the lamination surfaces of the Bi2Te3 layers and the metal layers in the laminate 13.
Table 1 shows the results of evaluation of the power factors of the respective elements 1 with respect to the change in inclination angle θ in the elements 1 (the elements 1 each have a metal layer of a Au layer, a Ag layer, a Cu layer, or an Al layer according to the type of the metal foil used therefor) formed using the respective metal foils. For example, in an element 1, with a metal layer being a Ag layer and the inclination angle θ being 30°, the electromotive voltage was 149 mV and the electrical resistance value was 0.28 mΩ. The power factor determined from those values was 918 (μW/(cm·K2)).







TABLE 1







[Change in power factor (μW/(cm · K2)) of


elements according to inclination angle θ]









Inclination angle θ (°)















0
15
30
45
60
75
90




















Au
0
336
648
562
298
80
0



Ag
0
518
918
760
408
98
0



Cu
0
502
860
718
380
92
0



Al
0
220
480
435
224
52
0










As shown in Table 1, the power factor was not obtained with respect to the elements with inclination angles θ of 0° and 90°, i.e. the elements in which the lamination surfaces of the Bi2Te3 layers and the metal layers were in parallel with or orthogonal to the direction in which the first and second electrodes opposed each other. On the other hand, the power factors were obtained with respect to the elements with inclination angles θ other than 0° and 90°, i.e. the elements in which the lamination surfaces of the Bi2Te3 layers and the metal layers were inclined with respect to the direction in which the first and second electrodes opposed each other. With respect to the elements with inclination angles θ of 15° to 60°, high power factors were obtained, specifically, it was at least 200 (μW/(cm·K2)) when the metal forming the metal layers was Al, at least 290 (μW/(cm·K2)) when the metal forming the metal layers was Au, and at least 380 (μW/(cm·K2)) when the metal forming the metal layers was Ag or Cu. That is, the elements with inclination angles θ of 15° to 60° made it possible to obtain high power factors, specifically at least 10 times that of an element having a π-type structure formed using Bi2Te3 as a thermoelectric material, which is utilized practically at present.
Example 2
In Example 2, elements that were different in thickness ratio between a metal layer and a Bi2Te3 layer were produced in the same manner as in Example 1. The thermoelectric power generation characteristics thereof were evaluated.
The elements were produced using copper foils, each of which had a thickness of 20 μm, as the metal foils (i.e. having Cu layers, each of which had a thickness of 20 μm, as the metal layers), with the thickness of each Bi2Te3 film formed on each surface of the copper foils being changed in the range of 0.05 μm to 4 μm. The inclination angle θ was fixed at 30°.
With respect to the elements thus produced, the power factors thereof were evaluated in the same manner as in Example 1. The results thereof are indicated in Table 2.







TABLE 2







[Change in power factor of element according to thickness ratio


between metal layer (20-μm thick Cu layer) and Bi2Te3 layer]









Thickness of Bi2Te3 layer (μm)

















4
2
1
0.5
0.25
0.2
0.1
0.08
0.05




















Cu layer:
5:1
10:1
20:1
40:1
80:1
100:1
200:1
250:1
400:1


Bi2Te3 layer


Power factor
148
250
437
679
852
871
753
645
403


(μW/(cm · K2))









As indicated in Table 2, when the thickness of the Bi2Te3 layer was in the range of 0.05 to 1 μm, i.e. when the thickness ratio between the Cu layer and the Bi2Te3 layer was in the range of Cu layer:Bi2Te3 layer=400:1 to 20:1, high power factors of at least 400 (μW/(cm·K2)) were obtained. Furthermore, when the thickness of the Bi2Te3 layer was in the range of 0.2 to 0.25 μm, i.e. when the thickness ratio between the Cu layer and the Bi2Te3 layer was in the range of Cu layer:Bi2Te3 layer=100:1 to 80:1 (the thickness ratio of the Bi2Te3 layers to the laminate was approximately 1%), high power factors of at least 800 (μW/(cm·K2)) were obtained.
Example 3
In Example 3, elements with different inclination angles θ from one another, each of which had a Cu layer as the metal layer, were produced in the same manner as in Example 1, and the thermoelectric power generation characteristics thereof were evaluated.
Elements were produced, in each of which 20-μm thick copper foils were used as the metal foils (i.e. 20-μm thick Cu layers were provided as metal layers), the thickness of each Bi2Te3 film that was formed on each surface of the copper foils was set at 0.1 μm (i.e. the thickness ratio between a Cu layer and a Bi2Te3 layer was fixed at a ratio of Cu layer:Bi2Te3 layer=100:1 in an element formed therewith), and the inclination angle θ was changed at 5° intervals from 15° to 60°.
With respect to the elements thus produced, the power factors thereof were evaluated in the same manner as in Example 1. The results are indicated in Table 3 below.







TABLE 3







[Change in power factor of elements according to inclination angle


θ (°), with thickness ratio between metal layer (Cu layer) and


Bi2Te3 layer being a ratio of Cu layer:Bi2Te3 layer = 100:1]









Inclination angle θ (°)


















15
20
25
30
35
40
45
50
55
60





















Power factor
495
688
811
871
870
815
723
617
502
389


(μW/(cm · K2))









As shown in Table 3, high power factors of at least 380 μW/(cm·K2) were obtained in all the samples. Particularly, higher power factors of at least 800 μW/(cm·K2) were obtained in the range where the inclination angle θ was 25° to 40°. That is, power factors obtained thereby were as high as at least 20 times an element having a π-type structure using Bi2Te3 as a thermoelectric material, which is utilized practically at present.
Example 4
In Example 4, in order to obtain a larger amount of thermoelectric power generation by increasing the area where the elements were mounted, a thermoelectric power generation device 41 as shown in FIG. 4 was produced. The type of metal that formed the metal layers of each element 1 was Cu, and Cu also was used for the connecting electrodes 43 and the extraction electrodes 44.
An alumina plate was used for the support 45, and each element 1 to be disposed on the alumina plate was produced in the same manner as in Example 1. In the element 1, the thickness of each Cu layer was 20 μm, the thickness of each Bi2Te3 layer was 0.2 μm (i.e. the thickness ratio between the Cu layer and the Bi2Te3 layer was Cu layer:Bi2Te3 layer=100:1), and the inclination angle θ was set at 30°. The size of the laminate 13 of the element 1 was set to a length of 50 mm, a width of 3 mm, and a thickness of 0.5 m. A Cu plate with a thickness of 0.5 mm was used for each of the connecting electrodes 43 and the extraction electrodes 44.
Fifteen elements 1 were prepared and the respective elements thus prepared were arranged on the support 45 at 1 mm intervals. As shown in FIG. 4, adjacent elements 1 were connected electrically in series with each other through a connecting electrode 43. In this case, the directions in which the Bi2Te3 layers were inclined in adjacent elements 1 were opposite to each other, so that the electromotive forces of the respective elements 1 generated due to the temperature difference were not cancelled with each other. The fifteen elements 1 were disposed in a range of approximately 60 mm×60 mm. The connecting electrodes 43 and the elements 1 as well as the extraction electrodes 44 and the elements 1 were connected electrically to each other by heat pressure bonding using a small amount of Bi (bismuth) pieces, respectively.
In the thermoelectric power generation device 41 thus produced, the electrical resistance value obtained between the extraction electrodes 44 was measured and was 22 mΩ.
Next, the back surface (the opposite surface to the surface on which the elements 1 were disposed) of the support 46 was maintained at 25° C. by water-cooling, and the opposite surface of each element 1 to the surface that was in contact with the support 46 was maintained at 40° C. with a ceramic heater that was attached thereto closely. As a result, the open end electromotive voltage obtained between the extraction electrodes 44 was 1.37 V. When being estimated from that value and the electrical resistance value measured above, the power factor of the thermoelectric power generation device 41 produced as described above was 845 (μW/(cm·K2), and it was possible to extract an electric power of up to 20 W.
As described above, the present invention makes it possible to obtain higher thermoelectric power generation characteristics as compared to the electric power generation methods, thermoelectric power generation elements, and thermoelectric power generation devices, in which conventional thermoelectric materials are used. The present invention improves the efficiency of energy conversion between thermal energy and electrical energy and has an effect of facilitating application of thermoelectric power generation to various fields and thus has an industrially high value.
Examples of promising applications include an electric generator that utilizes exhaust gas heat from automobiles or factories, and a small portable electric generator.
The present invention may be embodied in other forms without departing from the spirit and essential characteristics thereof. The embodiments disclosed in this specification are to be considered in all respects as illustrative and not limiting. The scope of the present invention is indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

## Claims

1. An electric power generation method using a thermoelectric power generation element for obtaining electric power from the element by applying a temperature difference in the thermoelectric power generation element,
wherein the element comprises a first electrode and a second electrode that are disposed to oppose each other, and
a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes,
the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately,
a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer: Bi2Te3 layer=400:1 to 20:1, and
lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to a direction in which the first electrode and the second electrode oppose each other,
the method comprising applying a temperature difference in a direction perpendicular to the direction in which the first electrode and the second electrode oppose each other in the element, so that electric power is obtained through the first and second electrodes.

2. The electric power generation method using a thermoelectric power generation element according to claim 1, wherein the inclination angle θ of the lamination surfaces with respect to the direction is 25° to 40°.

3. The electric power generation method using a thermoelectric power generation element according to claim 1, wherein the metal layer contains Cu, Ag, or Au.

4. The electric power generation method using a thermoelectric power generation element according to claim 1, wherein the metal layer contains Cu or Ag.

5. The electric power generation method using a thermoelectric power generation element according to claim 1, wherein the thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=100:1 to 80:1.

6. The electric power generation method using a thermoelectric power generation element according to claim 1, wherein the element has a power factor of at least 200 (μW/(cm·K2)).

7. The electric power generation method using a thermoelectric power generation element according to claim 2, wherein the metal layer contains Cu or Ag, and
the thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=100:1 to 80:1.

8. The electric power generation method using a thermoelectric power generation element according to claim 7, wherein the element has a power factor of at least 800 (μW/(cm·K2)).

9. A thermoelectric power generation element, comprising:
a first electrode and a second electrode that are disposed to oppose each other, and
a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes,
where the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately,
a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1,
lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to a direction in which the first electrode and the second electrode oppose each other, and
a temperature difference applied in a direction perpendicular to the direction in which the first electrode and the second electrode oppose each other in the element generates a potential difference between the first and second electrodes.

10. The thermoelectric power generation element according to claim 9, wherein the inclination angle θ of the lamination surfaces with respect to the direction is 25° to 40°.

11. The thermoelectric power generation element according to claim 9, wherein the metal layer contains Cu, Ag, or Au.

12. The thermoelectric power generation element according to claim 9, wherein the metal layer contains Cu or Ag.

13. The thermoelectric power generation element according to claim 9, wherein the thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=100:1 to 80:1.

14. The thermoelectric power generation element according to claim 9, wherein the element has a power factor of at least 200 (μW/(cm·K2)).

15. The thermoelectric power generation element according to claim 10, wherein the metal layer contains Cu or Ag, and
the thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=100:1 to 80:1.

16. The thermoelectric power generation element according to claim 15, wherein the element has a power factor of at least 800 (μW/(cm·K2)).

17. A method of producing a thermoelectric power generation element, the element comprising:
a first electrode and a second electrode that are disposed to oppose each other, and
a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes,
where the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately,
a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1,
lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to a direction in which the first electrode and the second electrode oppose each other, and
a temperature difference applied in a direction perpendicular to the direction in which the first electrode and the second electrode oppose each other in the element generates a potential difference between the first and second electrodes,
wherein the method comprises cutting out an original plate, in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately and a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1, in a direction that obliquely traverses lamination surfaces of the Bi2Te3 layer and the metal layer, and disposing the first and second electrodes on the laminate thus obtained so that the first and second electrodes oppose each other and a direction in which they oppose each other traverses the lamination surfaces at an inclination angle θ of 15° to 60°.

18. A thermoelectric power generation device, comprising:
a support plate and a thermoelectric power generation element disposed on the support plate,
where the element includes first and second electrodes that are disposed to oppose each other, and a laminate that is interposed between the first and second electrodes and that is electrically connected to both the first and second electrodes,
the laminate has a structure in which a Bi2Te3 layer and a metal layer containing Al, Cu, Ag, or Au are laminated alternately,
a thickness ratio between the metal layer and the Bi2Te3 layer is in a range of metal layer:Bi2Te3 layer=400:1 to 20:1,
lamination surfaces of the Bi2Te3 layer and the metal layer are inclined at an inclination angle θ of 15° to 60° with respect to a direction in which the electrodes of a pair oppose each other,
the element is disposed on the support plate in such a manner that a direction perpendicular to the direction in which the electrodes of a pair oppose each other agrees with a direction perpendicular to a surface of the support plate on which the element is disposed, and
a temperature difference is applied in the direction perpendicular to the surface of the support plate, so that electric power is obtained through the electrodes of a pair.

19. The thermoelectric power generation device according to claim 18, wherein the device includes at least two of the elements, and
the elements are connected electrically in series with each other through the electrodes.

20. The thermoelectric power generation device according to claim 18, wherein the device includes at least two of the elements, and
the elements are connected electrically in parallel with each other through the electrodes.